- #1
PhysicForFun
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Homework Statement
1. Find of the center of mass of the image below.
2. A human head weighs 8lbs, a human torso weighs 20lbs, and a human's hips and legs weighs 20lb. The headis 10m long, the torso is 25m long, and the hips and legs are 35m long. Find the center of mass of the body.
Homework Equations
X = Ʃ(m1x1...) / Ʃ(m1...)
Y = Ʃ(m1y1...) / Ʃ(m1...)
The Attempt at a Solution
Sorry I couldn't find a clear example. I have two different methods given to solve these problems, so I wouldlike clarification.
1. X = (2kg)(2m) + (.5kg)(5m) / (2kg + .5kg) = 2.6m
Y = (2kg)(2m) + (.5kg)(1m) / (2kg + .5kg) = 1.8m
So this one took the problems into point masses. Then solved for them.
X1= (2kg)(0m) + (2kg)(4m) / (2kg + 2kg) = 2m
Y1 = (2kg)(0m) + (2kg)(4m) / (2kg + 2kg) = 2m
X2 = (.5kg)(4m) + (.5kg)(6m) / (.5kg + .5kg) = 5m
Y2 = (.5kg)(0m) + (.5kg)(2m) / (.5kg + .5kg) = 1m
Y1 = (2kg)(0m) + (2kg)(4m) / (2kg + 2kg) = 2m
X2 = (.5kg)(4m) + (.5kg)(6m) / (.5kg + .5kg) = 5m
Y2 = (.5kg)(0m) + (.5kg)(2m) / (.5kg + .5kg) = 1m
2. X = (8lb)(10m) + (20lb)(25m) + (20lb)(35m) / (8lb + 20lb + 20lb) = 26.6m
Why does it just use the lengths instead of combining them, or even taking the half-way point? Maybe I misinterpretted the problem? I assumed this at first.
X = (8lb)(5m) + (20lb)(17.5m) + (20lb)(52.5m) / (48lb) = 30m
But I guess those shapes can't be divided perfetcly in half. But why is it not
X = (8lb)(10m) + (20lb)(35m) + (20lb)(70m) / (48lb) = 45m
This clearly is too big to be logical.
Could someone explain this? Or link me to some CoM problems that aren't using point masses. I think thepoint masses are sort of self-explanatory, but I can't find good examples otherwise.
Thanks.